A function of some extra corporeal blood treatment systems (ECBT systems), including hemodialysis, hemofiltration, hemodiafiltration, apheresis systems, etc., is the maintenance of the overall fluid balance between the fluid added to the patient and the fluid withdrawn from the patient. Ideally, this exchange will result in a net loss or gain of fluid to/from the patient that precisely matches the patient's treatment requirement. To achieve this, the ECBT may employ a volumetric fluid balancing system, of which a variety of different types are known. For example, see U.S. Pat. Nos. 5,836,908, 4,728,433, 5,344,568, 4,894,150, and 6,284,131, each of which is hereby incorporated by reference as if fully set forth in their entireties herein.
Fluid balancing mechanisms generally attempt to ensure that the total mass or volume of fluid pumped into, and removed from, the non-blood side of a filter or dialysis are equal. To provide for a desired differential between the net quantity removed/added, the inflow and outflow rates can be controlled to produce a net difference. This may be provided by regulating the rates of ingoing and outgoing pumps or by using a separate bypass, driven by a separate pump. In an example, such a bypass pump pumps at an ultrafiltration (“UF”) line rate which is added to the balanced withdrawal rate.
Gravimetric systems that balance flow by weighing mass from a source and collected fluid from the treatment device and comparing the two are known. Another approach is to measure incremental volume transfer. Hard plumbed or disposable lined balance chambers alternately fill and empty in a manner that assures equal and opposite volume exchange. Systems using this approach are balancing a single inlet fluid flow with an effluent stream. A second stream of fluid is frequently added to the extracorporeal circuit using an additional pump, or external IV pump. The volume of this second stream may be balanced by the isolated ultrafiltration (UF) pump in an attempt to maintain patient fluid balance. This approach is limited by the calibration inaccuracies of the additional or external pump and the isolated UF pump. These inaccuracies are acceptable at low flow rates. However, at higher flow rates the cumulative volumetric inaccuracies may not achieve the desired patient volumetric balance. Additionally, this approach requires an operator to independently set the pump rates to achieve the desired balance.